Field of the Invention
In order to prevent a printed circuit board from warping caused by the heat of molten solder, a long, plate-like warp-preventing means has been used in the past, which is arranged above a solder tank to be parallel in its longitudinal direction with the direction of movement of the board to be transferred and approximately at the same level as that of the molten solder in the solder tank where the molten solder is allowed to circulate strongly and overflow, whereby the means is brought into contact with the lower surface of the board being moved (Japanese Laid-open Utility Model Appln. No. Sho. 56-152161 .
In a conventional carrierless soldering apparatus, L-shaped warp-preventing plates are also used with a view to preventing a printed circuit board from warping caused by the heat of the molten solder or straightening a warped circuit board in such manner that both side edge portions of the board, in transverse direction to the direction of movement of the board, are allowed to rest on the plates. More precisely, this manner involves the steps of setting the warp-preventing plates in a position spaced by a given distance in longitudinal direction of the circuit board from both end portions of the circuit board, allowing the plates to descend by the action of a first driving means such as an air cylinder, moving the plates horizontally by the action of a second driving means such as an air cylinder above the position where the circuit board is to be supported on both edge portions of each plate, and elevating the plates by the action of the first driving means so that the board may be supported on both edge portions of the plates (Japanese Laid-open Patent Appln. No. Sho. 60-27188).
In the prior art warp-preventing means constructed as above, however, a problem arises in that some or all of the electronic parts and incidental lead wires mounted to a printed circuit board are brought into contact with the warp-preventing means, depending on the arrangement of the parts, thus disturbing the soldering treatment of the circuit board. In case of using the warp-preventing plates, another problem arises in that the soldering apparatus has to be provided separately with a first driving means for moving the warp-preventing plates up and down a the second driving means for moving the plates horizontally, thus rendering the structure of the soldering apparatus and the operation of the warp-preventing plates rather complicated. Further, printed circuit boards undergo upward or downward warping depending on the kind or nature thereof. Accordingly, a problem also arises in the conventional warp-preventing means which is effective exclusively for preventing downward warping of the circuit boards because it cannot prevent the occurrence of upward warping (or camber) of the circuit boards or cannot straighten upwardly warped circuit boards.
When printed circuit boards have undergone warping by the influence of heat on soldering, such warped circuit boards cannot uniformly be treated in the subsequent step where lead wires extending from the lower surface of the soldered circuit board are cut off uniformly by a rotating cutter positioned just below the circuit board along the path of its movement.
In case the circuit board has been warped downwardly during the soldering treatment so that the lead wires extending from the bottom of the circuit board cannot be cut off uniformly in the subsequent treatment, a warp-straightening device as shown in FIG. 12 is used in the prior art. In the soldering apparatus shown in FIG. 12 in which a solder tank is not shown for brevity and for simply depicting the structure of a device for straightening the downward warp of circuit boards, the apparatus involves a printed circuit board 3 having a front edge portion 3' relative to its moving direction shown by an arrow mark A, electronic part 3c and lead wires 3d, a conveying device 4, a cutter 58 with a circular cutting blade 59 and one or more of the warp-straightening devices 60 and 70. In this drawing, the devices 60 and 70 are used after the soldering treatment and prior to the treatment for cutting the lead wires extending from the bottom of the board 3. In the device 60 for straightening the downward warp of the circuit board 3, a rotary shaft 61 has radially arranged plural pins 63 and is mounted on a support 62. In the device 70 used for a similar purpose, a rotary shaft 71 has a wheellike rotary plate 72 and is mounted on a support 73. By the use of the warp-straightening device 60 or 70, the downward warp of the circuit board formed as a result of the thermal deformation of the circuit board during the soldering treatment can properly be straightened in such manner that the rotary pins 63 and/or the rotary plate 72 are/is brought into contact with the lower surface of the circuit board whereby a strong upwardly acting force is exerted on the downwardly warped portion of the board to flatten it. However, these devices 60 and 70 involve some defects; firstly, upwardly warped or cambered circuit boards cannot be flattened by the use of these device( 60 and 70. Secondly, the device 60 or 70 is interposed between a solder tank and a cutter and installed at a fixed position, so that the device cannot be used for preventing the formation of any warp in circuit boards during the soldering treatment. Further, when the circuit boards are once warped during the soldering treatment, correction of the warp cannot completely be made by the use of the device 60 or 70 so that the lead wires 3d may be cut slantingly or in some cases may not be cut off uniformly on account of vibration caused by the rotation of the cutter 58.
In order to prevent completely circuit boards from warping during the soldering treatment, the warp-preventing device should be movable together with the circuit board moving along the predetermined path in the apparatus and continuously support the boards in a flattened state until the soldered circuit board is cooled. Under the circumstances, there is a great demand for developing a new device for correcting warp of printed circuit boards which overcomes the drawbacks in the prior art devices.